In the oil and gas industries, coiled tubing refers to a very long metal pipe supplied spooled on a large reel. It is used for interventions in oil and gas wells and sometimes as production tubing in depleted gas wells. A relatively modern drilling technique involves using coiled tubing instead of conventional drill pipe. Instead of rotating the drill bit by using a rotary table or top drive at the surface, it is turned by a downhole mud motor, powered by drilling fluid pumped from the surface.
FIG. 1 illustrates generally a coiled tubing setup 5. Coiled tubing 7 is fed from a reel 8 into a coiled tubing injector 10 which effectively powers the tubing into a wellhead 12. The end of the coiled tubing string 7 can be outfitted with numerous downhole tools including drill bits and other related drilling equipment. The “gooseneck” 9 is the angled piece above the coiled tubing injector 10 which guides the coiled tubing string 7 and allows a bending of the coiled tubing string 7 to allow it to enter and pass through the injector 10. It is what guides the coiled tubing string 7 from the reel 8 and directs the tubing from an upwards angle and turns it to a vertically downward extending direction into the injector 10 and through a blow-out preventer (BOP) stack into the wellhead 12. The injector 10 and gooseneck 9 are connected together and are suspended by a crane or similar lifting methods for coiled tubing operations.
Oil and gas well drilling is typically performed using precise computerized methods to adjust instantaneously to any changes, faster than a human can process. Total human control in the past has led to damage to drill bits or the casing, and the weight of the coiled tubing string above it can force the coiled string into a “runaway” or uncontrolled descent. For example, too high of a drill rate does not allow for proper degradation of larger pieces of plugs or other materials, which then clog the pathway and restrict movement, and can damage an entire coiled tubing drillstring. Drilling is an extremely skilled profession and human operators may require years of training.
What is needed then is a control system for precisely maintaining rates of moving a coiled tubing string within a wellbore in various applications.